Gitlab: grab the last succesful build artifact from CI

Uo to now, we can’t just download the latest succesful build of a project (see #4768). Therefore, i’m using the API to fetch an ordered list of all project builds in state “success” and filter for the last one. Using this ID, we can download the artifact via API. For more details, check this file. It needs some vars to work: BASE_URL PRIVATE_TOKEN PROJECT, you’ll find more info in the project README.